Does the DCA only produce single deviants or double ones? I feel like it should produce double deviants if both parents are eligible with both pattern and color, but just checking to make sure. It just feels kind of unfair to only get a single deviant using this item when you could get a double.

Short answer: yes, a 100% DCA will produce a double deviant as long as your two parents are deviants, one with color pattern and one with skin pattern. 

For exemple, if you are referring to the double deviant you bought from me, Hallucinogens, that you mated with winlemasity, which is not deviant, adding a DCA won't help you get a double deviant. 

Also, see a question I asked related to your question : https://beastkeeper.com/thread/12298/1#97981

Oh! Interesting. I suppose it'll be hard because apparently only color deviants like me and the pattern ones don't... :,,,/ this makes life a bit harder, argh.

Sure it is hard. I have been working for a year now on this type of project. 

Thank you a ton for telling me about the rule, though. I've adjusted my breeding plans accordingly, lol

Now I actually have a chance at em instead of just blindly breeding like I was before.

Good luck and enjoy ;)